(57) Disclosed herein are compounds of the general formula X-A-B-C'-Y wherein X is lower alkanoyl optionally substituted with one or two hydroxyls, A is an amino acid residue having a lower alkyl side chain, e.g. Ala or Val, B is a non-peptide linking unit, e.g. statyl or PheΨ[CH₂NH]Leu, C' is an amino acid residue of α-amino-β-cycloalkylpropionic acid, glutamic acid or α-aminoadipic acid, or a related derivative thereof, and Y is alkoxy, cycloalkoxy, a substituted alkoxy or a substituted amino group, e.g. cycloalkylalkoxy, arylalkoxy, alkylamino, (cycloalkyl)alkylamino or arylalkylamino. The compounds inhibit the activity of human immunodeficiency virus (HIV) protease and interfere with HIV induced cytopathogenic effects in human cells. These properties render the compounds useful for combating HIV infections.